Thermoelectrically cooling electronic devices

ABSTRACT

A thermoelectric cooler utilizing superlattice and quantum-well materials may be deposited directly onto a die using thin-film deposition techniques. The materials may have a figure-of-merit of greater than one.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to thermoelectric cooling of electronicdevices.

2. Description of the Related Art

Heat generated during processor operation may adversely effect theprocessor's performance and may damage the processor. Thus, it isdesirable to keep processors and other heat generating electronicdevices cool. Cooling processors may increase processor performance anddecrease the potential for damage.

Traditional methods of cooling may either be impractical for use withsmall devices, such as microprocessors, or may be practical butinefficient. For example, cooling a processor by conduction may notproduce sufficiently low temperatures due to resistance from thecomponents used in the cooling process. Moreover, refrigeration coolingmay produce sufficiently cool temperatures but the volume of coolingsolution and amount of accompanying hardware do not make this systempractical for use with small devices, such as a microprocessor.

Thermoelectric cooling, for example by a Peltier device, may bepractical for use in small electronic devices because the Peltierdevices are compact. Generally, when a current is applied to aPeltier-type thermoelectric cooling device, it will absorb-heat from onesurface of the electronic device and release the heat somewhere else.Traditional thermoelectric coolers however may require a lot of power.This may make these coolers inefficient. Especially when the devicebeing cooled is battery powered, such coolers may be impractical.

Thus, there is a need for more efficient ways of thermoelectric coolingelectronic devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a greatly enlarged front elevational view of one embodiment ofthe present invention showing the packaged and cooled die; and

FIG. 2 is a reduced front elevational view of a semiconductor wafer inthe course of fabrication.

DETAILED DESCRIPTION OF THE INVENTION

An electronic device 10, illustrated in FIG. 1, may include a die 18utilizing a package 22. According to one embodiment of the invention,the package 22 may be an organic land grid array package (OLGA).However, other packaging techniques may be utilized. A layer ofunderfill between the package 22 and the die 18 couples the die 18 tothe package 22. Solder bumps 24 may be used to electrically andmechanically couple the die 18 to a circuit board (not shown) usingsurface mount techniques.

A thermoelectric cooler (TEC) 16 may be formed directly on the die 18.The TEC 16 may be made from thin-film superlattice and quantum-wellstructures. Such structures may use materials having a figure-of-meritor ZT value of greater than one. The figure-of-merit ZT is expressed asfollows:

ZT=(α² σ/K)T

where

α is the Seebeck coefficient;

σ is the electrical conductivity;

K is the sum total of the lattice and electronic components of thermalconductivity; and

T is the temperature (°K).

In general, superlattices may be comprised of alternating thin layers ofP-type and N-type semiconductor materials. These alternating layers maybe barrier layers and quantum-well layers. In one embodiment of thepresent invention, alternating layers of Bi₂Te₃ and Sb₂Te₃ may beutilized in forming a superlattice quantum-well structure. At 300 K, theZT of these layers may be from about 1.7 to about 2.3. The alternatinglayers may be of the same thickness or the Sb₂Te₃ layer may be madethicker than the Bi₂Te₃ layer. The composite thickness of thealternating layers may be about 500 microns and the individual layersmay be from 20 to 200 Angstroms in thickness.

Thermoelectric coolers 16 utilizing superlattice and quantum-wellstructures may have higher ZT values, and thus, they may produce a moreefficient thermoelectric cooler than the traditional thermoelectriccoolers. With an increased cooling efficiency comes a decrease intemperature and hence a faster electronic device. Moreover, depositingthe TEC 16 directly onto the die 18 may result in a substantialreduction in temperature at the die 18/TEC 16 interface. As a result,the leakage power consumption of the die 18 may also be reduced.

The thin-film TEC 16 may be directly deposited onto the die 18 usingtechniques such as molecular beam epitaxy (MBE) and metal organicchemical vapor deposition (MOCVD). MBE and MOCVD are vapor depositiontechniques used to deposit layers of materials on a substrate at theatomistic level. The materials deposited onto the die may be anymaterials with sufficiently high ZT values such as Bi₂Te₃/Sb₂Te₃, as oneexample.

A thermal interface material 14 may be positioned between thethermoelectric cooler 16 and a heat pipe 12. Thus, as the TEC 16 drawsheat away from the die 18, the heat pipe 12 in turn may remove heat fromthe thermoelectric cooler 16 and release it away from the die 18. Inthis way, the heat produced by the die 18 may be continually removed bymaintaining a temperature gradient across the TEC 16. Thus, the die 18may be kept cool, which may prevent it from sustaining damage whileimproving its performance.

In one embodiment of the present invention, the die 18 and thethermoelectric cooler 16 may formed from a semiconductor wafer 26, asshown in FIG. 2. The semiconductor wafer 26 may have a front side 32 anda back side 30. The back side 30 of the wafer 26 receives the depositionof alternating layers of high ZT material using a thin-film depositiontechnique such as MBE or MOCVD. The wafer 26 may be singulated into diceand a die 18 may be attached with its front side 32 coupled to thepackage 22.

Because MBE or MOCVD may be employed to deposit the thermoelectriccooler material, there is no need for the use of thermal interfacematerial between the TEC 16 and the die 18. That is, because thethermoelectric cooler material may be deposited on the wafer 26 at theatomistic level, there is no need for an interface material. Moreover,because the TEC 16 and the die 18 are effectively integral there islittle, if any, interfacial resistance to thermal conduction. Thus, thedie 18 may maintain a cooler operating temperature.

Deposition of the thermoelectric cooler material may occur in much thesame way other layers are deposited on the front side 32 of a waferduring conventional integrated circuit fabrication process. Thus,thermoelectric cooler 16 fabrication steps may take place duringfabrication of the wafer that results in the die 18. Moreover, due toits compactness, the thin-film TEC 16 may contribute to a compactpackage height that is ideal for use in small electronic devices.

Using the techniques described herein, junction temperatures more thanfifty percent lower than that achieved with conventional coolingtechniques may be achieved in some embodiments. The temperature of thecold junction of the thin film TEC 16 may be much lower than thatachieved with traditional thermoelectric cooling with the same heatremoval. For example, based on modeling, temperatures of approximately50° C. may be achieved. At such temperatures, the leakage powerconsumption of an electronic device 18, such as a processor, may besignificantly reduced.

Moreover, the savings in leakage power consumption may be sufficient tocompensate for or to balance the power used for thermoelectric cooling.Thus, improved results may be achieved either without increasing orwithout substantially increasing the power consumption of the electronicdevice and cooling system. Moreover, because a thermal interfacematerial is dispensed with, the temperature of the die surface iseffectively that of the junction of the TEC 16.

While the present invention has been described with respect to a limitednumber of embodiments, those skilled in the art will appreciate numerousmodifications and variations therefrom. It is intended that the appendedclaims cover all such modifications and variations as fall within thetrue spirit and scope of this present invention.

What is claimed is:
 1. A method comprising: fabricating a die with afront side and a back side; and directly depositing a thermoelectricmaterial on the back side of the die.
 2. The method of claim 1 includingdepositing a thermoelectric material with a figure-of-merit that isgreater than one.
 3. The method of claim 2 including depositing athermoelectric material that has a superlattice and quantum-wellstructures.
 4. The method of claim 3 wherein depositing includesdepositing alternating layers of Bi₂Te₃ and Sb₂Te₃.
 5. The method ofclaim 3 wherein depositing includes forming a thermoelectric material ofabout 500 microns thickness.
 6. The method of claim 1 includingdepositing using a thin-film deposition technique.
 7. The method ofclaim 6 wherein depositing includes using molecular beam epitaxy.
 8. Themethod of claim 6 wherein depositing includes using metal organicchemical vapor deposition.
 9. The method of claim 1 including depositingthe thermoelectric material during fabrication of the die.
 10. Anelectronic device comprising: a die with a front side and a back side;and a thermoelectric cooling material directly deposited on the backside of the die.
 11. The device of claim 10 wherein the thermoelectricmaterial has a figure of merit that is greater than one.
 12. The deviceof claim 11 wherein the thermoelectric material has a superlattice andquantum-well structures.
 13. The device of claim 12 wherein thesuperlattice and quantum-well structures includes alternating layers ofBi₂Te₃/Sb₂Te₃.
 14. The device of claim 12 wherein the thermoelectriccooling material is 500 microns thick.
 15. The device of claim l0wherein said die includes a processor.
 16. A processor comprising: a diedirectly coated on its back side with a thermoelectric cooling materialhaving a figure-of-merit of greater than one; and a package coupled tothe die.
 17. The processor of claim 16 wherein the thermoelectricmaterial has a superlattice and quantum-well structures.
 18. Theprocessor of claim 17 wherein the superlattice and quantum-wellstructures includes alternating layers of Bi₂Te₃/Sb₂Te₃.
 19. Theprocessor of claim 16 wherein the thermoelectric cooling material isabout 500 microns thick.